[libc++] Improve the verbosity of configuration errors when a compiler flag is...

[libc++] Improve the verbosity of configuration errors when a compiler flag is not supported (#66379)

If an assertion fails during the configuration of the test suite because
the compiler doesn't support a flag (or the compiler configuration is
borked entirely), we will now print additional context to help debug the
issue instead of just saying "The compiler doesn't support the flag" and
bailing.